The nitrogen element can have an atomic value varying from monovalent to pentavalent, and there are many types of nitrogen oxides (usually in a gas state under a normal temperature and normal pressure), such as nitrous oxide (N2O), nitrogen monoxide (NO) and nitrogen dioxide (NO2). Also, a nitrogen oxide can be easily reacted with another nitrogen oxide, thereby giving a new nitrogen oxide. Accordingly, a system containing the nitrogen oxide tends to be complicated, in terms of the type and amount of the nitrogen oxides involved. For example, upon manufacturing NO through an ammonia oxidation method, which is a popular industrial manufacturing method thereof, N2O, NO2, and N2 are generated as by-products in addition to the desired NO, and mixed in the NO gas as impurities.
To increase the purity of a nitrogen oxide, or to refine the same, distillation may be performed. When executing the distillation of the nitrogen oxide, however, the step often has to be performed under an ultra-low temperature, mainly because the boiling points of N2O and NO are significantly low (boiling point of N2O is −89° C., and NO −152° C.). The nitrogen oxide is what is known as an endothermic compound, and executing the ultra-low temperature step is problematic from the viewpoint of safety, because it is known that the condensed nitrogen oxide may even explode from an impact, depending on the condition. Especially, the impact sensitivity of liquefied NO with respect to explosion probability is close to that of nitroglycerin, and therefore purification of NO by distillation presents a serious problem in the aspect of safety.
Meanwhile, another known method of purifying a nitrogen oxide is supplying a raw gas (source gas) for the nitrogen oxide under a predetermined condition so as to flow through a column loaded with an adsorbent such as zeolite. By this method, a predetermined component in the source gas is adsorbed by the adsorbent, to be thereby separated from unabsorbed components. The separation technique of N2O and NO utilizing the zeolite adsorbent can be found, for example, in the patent document 1 cited below. When purifying a target nitrogen oxide with the separation technique employing the zeolite adsorbent, however, the larger number of nitrogen oxide species the source gas contains, the more types of zeolite adsorbents have to be employed, and in many cases the number of columns also has to be increased, which makes the purification system complicated. Such technique often fails to efficiently purify a nitrogen oxide (often contained in a multicomponent system before the purification).
Patent document 1: JP-A-2004-10391